1. Field of the Invention
The present invention relates to treatments for neurological disorders and, more particularly, to neurotrophic factor peptides.
2. Description of the Related Art
Brain derived neurotrophic factor (BDNF), a member of the neurotrophin family that also includes nerve growth factor (NGF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5), promotes neuronal survival, differentiation, and synaptic function through the signaling of its receptor tropomyosin-related kinase-B (TrkB). Brain derived neurotrophic factor is of particular therapeutic interest because its expression levels are altered in many neurological disorders. A neurotrophic factor starvation, including NGF and BDNF deficiency, that begins in the early stages of Alzheimer disease (AD) and ultimately causes neuronal degeneration, cell death, and loss of cholinergic neurotransmission in the late stages of the disease has been reported. Additionally, the expression levels of BDNF are also reported to be reduced in Parkinson's disease (PD), depression, and stress. Conversely, autism spectrum disorders (ASDs) are characterized by an increase in BDNF levels. Thus, modulation of BDNF levels in these neurological disorders as a potential therapeutic approach is suggested.
Brain derived neurotrophic factor plays important roles in plasticity of several regions of the central nervous system (CNS) during development, adulthood, and ageing. The multiple roles of BDNF depend on functional and morphological changes, like protein phosphorylation, generation of new neurons, and cytoskeletal reorganization of dendritic spines. In hippocampal neurons, cyclic adenosine monophosphate (cAMP) controls BDNF-induced TrkB phosphorylation and dendritic spine formation by modulating the signaling and trafficking of TrkB.
Brain derived neurotrophic factor shares about 50% amino acid identities with NGF, NT-3 and NT-4/5. Each neurotrophin consists of a non-covalently-linked homodimer and contains a signal peptide following the initiation codon and a proregion containing an N-linked glycosylation site. Initially neurotrophins are produced as proneurotrophins (molecular weight-30 KDa), that are cleaved by enzymes such as prohormone convertases e.g. furin generating the mature neurotrophin (molecular weight of 14-26 KDa). Proneurotrophins have distinct biological activities and binding characteristics.
The immature form of BDNF is called proBDNF, and consists of 247 amino acids (in comparison with the mature form of BDNF that has 119 amino acids). This proneurotrophin binds a different receptor, known as low affinity p75NGFR, a member of the tumor necrosis factor (TNF) receptor super family and minimally binds Trk receptors. Brain derived neurotrophic factor and proBDNF are reported to have opposite effects. The activation of p75NGFR receptor can cause apoptosis in a variety of systems; instead, the activation of the TrkB receptor alone, as mentioned above, can promote differentiation, survival, and/or neuronal plasticity. Nevertheless, in physiological conditions neurons probably do not have high amounts of available extracellular proBDNF, because the endogenous proBDNF is rapidly converted to BDNF.
Pharmacologic modulation of BDNF levels has been suggested as a potential treatment strategy for human neurodegenerative diseases. A number of properties limit the therapeutic use of BDNF; the compound has a very short (less than 1 min) plasma half-life, and it has poor blood brain barrier (BBB) and intraparenchymal penetrations. Thus, the there is a need in the art for molecules, such as small peptides that could mimic or modulate the functions of BDNF, and have higher permeability and stability than BDNF itself. The general lack of success of neurotrophic factors in clinical trials (due to low stability in plasma and low permeability through the BBB) has led to the idea that low molecular weight neurotrophic factor mimetics can serve better as pharmacological agents.
However, the low plasma stability and low BBB permeability because of its moderately large size and ionic charge practically precludes the use of this neurotrophic factor as such for therapeutic usage, at least via peripheral administration. For instance, in a phase III clinical trial for the treatment of amyotrophic lateral sclerosis (ALS), a daily subcutaneous administration of BDNF offered no clinical benefit. Alternatively, direct administration of BDNF into the CNS to achieve beneficial neurotrophic effects may be a promising approach; however, there are also some considerations with this strategy that need to be taken into account. The CNS is composed of extremely delicate neural tissue sustained in a tightly controlled homeostatic environment, and direct intraventricular or intrathecal administration of a growth factor can cause undesirable effects. Direct administration of BDNF into the CNS has been reported to cause weight loss, dysaesthesias (impairment of sensation), and in some cases, pain. Direct administration into CNS can be a better alternative if effective concentrations of the neurotrophic factor can be achieved at precise sites of degenerating neurons, while limiting the spread to distant sites to avoid undesirable effects.
One method to attain this can be gene delivery via adeno-associated viral vectors (AAVs). However, this approach is now in evaluation, and it requires additional improvements to guaranty the safety of the patients. Other alternatives include non-pharmacologic approaches for BDNF augmentation such as exercise and diet modulation. Physical exercise increases BDNF levels in the hippocampus and the cortex, and may enhance learning and memory, synaptic plasticity, and neurogenesis. Caloric restriction also affects the levels of BDNF. However, changes in BDNF expression levels due to exercise or caloric restriction are low as compared with the direct administration of the neurotrophic factor by infusion. Epigenetic modulation of gene transcription, as an alternative approach, can be achieved through direct methylation of DNA or by post-translational modification of histones, which can either repress or promote gene transcription. Fear conditioning has been shown to differentially regulate the expression of BDNF mRNAs, following BDNF DNA methylation. Drugs that are able to increase BDNF levels in the brain include antidepressants, e.g., lithium, that is able to increase 30% BDNF concentrations in serum, and ampakines that increase BDNF and improve stabilization of LTP and long-term memory in a mouse model of Huntington's disease. Whether these drugs induce sufficient changes in BDNF levels to be useful for human diseases remains to be determined. Also remaining to be evaluated are the mechanisms that these drugs employ to modulate BDNF expression, since most of them can also activate alternative cellular signaling pathways, generating a complex mechanism of action.
In order to exploit the therapeutic value of BDNF, some peptide mimetics have been identified. For the selection of the group of molecules, in silico screening (computational modeling) with a BDNF loop-domain pharmacophore was, followed by in vitro screening in mouse fetal hippocampal neurons. These small molecules (LM22A1 to 4) showed neurotrophic activity specific to TrkB versus other Trk family members (Massa, Yang et al. 2010). However, these molecules were not peptides in chemical structure, so they may have disadvantages such as toxicity or low solubility that could hinder their development as clinical drugs.